In lithographic methods for producing microstructured or nanostructured components for microelectronics or microsystem engineering, structures on a mask, a so-called reticle, are imaged on a semiconductor material in order thus to produce conductor tracks and semiconductor components. In order to be able to produce structures in the nanometer range on the semiconductor material, the structures have to be produced and positioned highly precisely on the reticle. In order to monitor the quality and measure the dimensional accuracy of the structures on the reticle, use is made of measurement methods and measuring microscopes, as are described in DE102009019140A1 and US 2014/0307949A1, for example.
Establishing the position of the masks is based upon an interferometric length measurement. To this end, special adjustment marks on a mask are captured in respect of the position thereof by use of a microscopic image. The individual adjustment marks or structure elements of the mask are successively driven into the center of the image field by way of a positioning stage and the position of the respective adjustment marks is established. Thereupon, the distance from the adjustment mark measured previously is established by determining the path traveled by the positioning stage between the measurements. The path traveled by the positioning stage is established by use of an interferometric measurement.
In order to facilitate highly accurate measurements of the reticles, it is necessary to very precisely know and optionally compensate the influence of variations in the ambient conditions (e.g., temperature, air pressure, humidity, . . . ) on the mask to be measured and on the measuring microscope used to carry out this measurement. A (natural or artificially produced) change in these ambient conditions leads to a change in the optical medium within an imaging optical unit of the measuring microscope which, in general, consists of a multiplicity of lens elements with air interstices. These interstices may also be purged with nitrogen. Furthermore, the mechanical hold of the lens elements may depend on ambient conditions such as air pressure and air temperature, for example. Consequently, the scale ratios of the projection exposure apparatus change in the case of a temporal modification of the ambient conditions. A typical change in the optical path in a measuring microscope on account of air pressure variation lies at 20 nm/mbar. Such a change appears to be relatively small, but it is very important for highly accurate measuring appliances, as are used, for example, for measuring photolithographic masks with structures in the nanometer range.
It is known that variations in the ambient conditions cause changes in the focal position in the measuring microscope. This effect is relevant, in particular, if the measuring microscope uses the focus position, for example to carry out a height measurement of a reticle for photolithographic applications. A method for establishing and compensating this effect is described in (file reference DE 10 2016 204 535), for example.
Furthermore, changes in the ambient conditions have an influence on the magnification of the optical imaging in the measuring microscope. As a consequence, the distances between two structures to be measured change in the image field of the measuring microscope when the ambient conditions vary. Such variations typically are of the order of up to 1.5-2 ppm; they therefore represent a noticeable effect, which leads to falsifications of the measurement results, for highly accurate measuring appliances, as are used, for example, for measuring structures on reticles. Until now, a satisfactory solution for compensating this effect had not been known.